The present invention relates generally to the field of seismic data processing, and more particularly to a method for determining the vertical depth and the vertical P-wave and S-wave velocities to estimate anisotropic parameters via prestack depth migration of P-P and P-S seismic data in the presence of VTI anisotropy.
A seismic survey begins with the sending of acoustic energy from a source such as an explosion, air gun, or seismic vibrator into the earth in the form of a non-polarized, omni-directional wavefield, and the recording by spaced-apart seismic sensors of acoustic energy which is reflected or refracted back from inhomogeneities or discontinuities in subsurface layers. Seismic energy propagates through the earth as compresional P-waves and shear S-waves. P-waves propagate in-line with the direction of travel of the source acoustic energy, and S-waves flow transversely and horizontally (xe2x80x9cSH wavexe2x80x9d) as well as vertically (xe2x80x9cSVxe2x80x9d wave) with respect to the direction of travel of the source acoustic energy. At any subsurface discontinuity, a P-wave may convert to an S-wave. If the conversion happens only once from an incident P-wave to a reflected S-wave, the converted wave is referred to as a P-S converted wave, C-mode converted wave, or C-wave.
Referring to FIG. 1, a thick uniform isotropic layer of thickness xe2x80x9czxe2x80x9d is shown with an acoustic energy source xe2x80x9cSxe2x80x9d and a seismic detector xe2x80x9cRxe2x80x9d which are a distance of xe2x80x9cxxe2x80x9d apart. The distance xe2x80x9cxxe2x80x9d is referred to as the offset between the source xe2x80x9cSxe2x80x9d and the detector xe2x80x9cRxe2x80x9d. A compression or P-wave 11 originates from the source xe2x80x9cSxe2x80x9d, and is reflected from the bottom of the layer 10 at a horizontal distance of x/2 from the source xe2x80x9cSxe2x80x9d. The reflected acoustic energy wave is a P-wave 12 that is sensed and recorded by detector xe2x80x9cRxe2x80x9d, which may be a hydrophone or geophone. A further P-wave 13 originates from the source xe2x80x9cSxe2x80x9d, and is reflected from the bottom of layer 10 a horizontal distance of xe2x80x9cs+xcxe2x80x9d from the source xe2x80x9cSxe2x80x9d. P-wave 13 undergoes a conversion to a shear or S-wave 14 upon reflection from the bottom of the layer 10. As before stated, since the conversion occurs only once, the converted wave may be referred to as a C-mode converted wave or C-wave.
Subsurface layers which are isotropic exhibit the same velocity of propagation of acoustic energy in all directions. Other subsurface layers are anisotropic in that the velocity of propagation of acoustic energy is azimuth dependent. Flat-lying polar anisotropic (xe2x80x9cVTIxe2x80x9d) subsurface layers, also referred to as transversely isotropic media with vertical symmetry, give rise to only one C-mode reflection.
Conventional seismic processing relies heavily on a stack (or average) of seismic traces from a common midpoint (xe2x80x9cCMPxe2x80x9d) gather to reduce coherent and incoherent noise in a seismic section. The stacking approach is generally satisfactory for single mode seismic data (P-wave, S-wave), but often fails when applied to converted mode (C-wave) data due to the asymmetrical travel paths. Data for a true common reflection point (xe2x80x9cCRPxe2x80x9d), which for C-wave reflections is a common conversion point gather (xe2x80x9cCCPxe2x80x9d), is required.
Reflection data of all types, whether P-wave or S-wave, must be corrected for irregular time delays. As reflection events are detected by seismic detectors increasingly distant from the source xe2x80x9cSxe2x80x9d, the arrival time of the reflected signals becomes increasingly long. Such a systematic shift to longer reflection times due to increasing source-detector offsets is generally referred to as normal moveout or NMO. It is well known that normal moveout causes errors in determining compressional and shear wave velocities. If such errors remain uncorrected, stacked amplitudes of seismic events will be misaligned, and the behavior of reflecting interfaces between subsurface layers will be misrepresented.
To overcome the above maladies, methods for establishing and updating velocity models have become two important steps for multicomponent P-P and P-S prestack seismic depth imaging. Five anisotropic processing parameters are commonly used in multicomponent data processing: the vertical P-P velocity vpo, the vertical P-S velocity vso, the vertical depth zo, and the parameters xcex4 and "sgr" as defined in the articles, xe2x80x9cWeak elastic anisotropyxe2x80x9d, by L. Thomsen, Geophysics, vol. 51, pp. 1954-1966 (October 1986) and xe2x80x9cParameter estimation for VTI media using PP and PS reflection dataxe2x80x9d, by Ilya Tsvankin and Vladimir Grechka, Proceedings of the 71st Annual International Meeting of the Society of Exploration Geophysicists (Copyright 2001). The anisotropic parameter xcex4 is defined by:       δ    =                                        (                                          C                13                            +                              C                44                                      )                    2                -                              (                                          C                33                            -                              C                44                                      )                    2                            2        ⁢                              C            33                    ⁡                      (                                          C                33                            -                              C                44                                      )                                ,
where Cii are the components of the 6xc3x976 symmetric elastic modulus matrix relating the stress components of a linearly elastic material to a linear combination of the strain components. The anisotropic parameter "sgr" is defined by:       σ    =                            (                                    v              po                                      v              so                                )                2            ⁢              (                  ϵ          -          δ                )              ,
where the anisotropic parameter xcex5 is defined by:   ϵ  =                              C          11                -                  C          33                            2        ⁢                  C          33                      .  
The uncertainty caused by the presence of anisotropy in the estimation of vertical velocity and depth is addressed in Tsvankin and Grechka (2001). Unfortunately, no prior method is known which is successful in overcoming errors in determining such anisotropic processing parameters in the presence of a VTI anisotropic subsurface layer.
The present invention is directed to a method of depth consistent joint velocity inversion for substantially overcoming errors in estimating the anisotropic parameters vso, vpo, xcex4, "sgr", and the vertical depth z0 in a VTI anisotropic subsurface layer.
A process for updating velocity determinations for anisotropic P-P and P-S prestack depth migration in a transversely isotropic media with vertical symmetry (VTI).
In one aspect of the invention, upon corresponding reflections from P-P and P-S waves being identified, a depth consistent image gather is formed and a joint velocity inversion in depth is performed to estimate vertical depth zo, vertical P-wave velocity vpo, and vertical S-wave velocity vso.
In a second aspect of the invention, an isotropic depth migration using the above estimates of vso, vpo, and zo is performed by scanning focusing velocities from P-P depth consistent image gathers to determine an isotropic depth zpp.
In a third aspect of the invention, anisotropic parameters xcex4 and "sgr" are estimated based upon the above estimates of vpo, vso, zo, and zpp.
In a fourth aspect of the invention, the above estimates of vpo, vso, zo, and zpp, and the anisotropic parameters xcex4 and "sgr" are used to refine the estimates vso, vpo, and zo.
In another aspect of the invention, VTI anisotropic parameters may be determined (whether or not well log data is available) to produce reliable P-P and P-S depth consistent imaging.